1. Field of the Invention
The present invention relates to a recording medium capable of high-density recording, a method of manufacturing a recording medium, and a recording apparatus.
2. Description of the Related Art
Information handled by users is being markedly increased by drastic improvement in functions of information apparatuses such as personal computers. Under the circumstances, demands for an information recording-reproducing apparatus with a drastically improved recording density are being made higher and higher. In order to improve the recording density, it is necessary to miniaturize the size of a single recording cell or a recording mark, which constitutes the writing unit of recording on the recording medium. However, the miniaturization of the recording cell or the recording mark faces a series difficulty in the conventional recording medium.
For example, in a magnetic recording medium such as a hard disk, a polycrystalline material having a wide grain size distribution is used for forming the recording layer. However, the recording is rendered unstable in the recording layer formed of small polycrystalline grains because of thermal fluctuations of the crystal. Therefore, if the recording cell is small, recording is rendered unstable and noise generation is increased, though a problem is not generated in the case where the recording cell is large. The unstable recording and the increased noise generation are brought about because, if the recording cell is small, the number of crystals contained in the recording cell is rendered small and interaction among the recording cells is rendered relatively large.
This is also the case with an optical recording medium using a phase change material. Specifically, recording is rendered unstable and medium noise is increased in a recording density not lower than several hundred gigabits per square inch, in which the recording mark size is substantially equal to the grain size of the phase change material.
In order to avoid the difficulties pointed out above, proposed in the field of the magnetic recording is a patterned media, in which a recording material is divided in advance by a non-recording material so as to carry out a recording-reproducing by using a single recording material particle as a single recording cell, as disclosed in, for example, U.S. Pat. Nos. 5,587,223, 5,956,216 and 6,162,532.
However, a lithography technique is used in the conventional method of forming the structure in which the recording material particles are isolated. It is certainly possible for optical lithography to cope with the requirement of a high recording density in terms of throughput because single step exposure can be employed. However, the optical lithography is hard to process recording cells sufficiently small in size. Electron beam lithography or a focused ion beam permit fine processing of about 10 nm. However, it is difficult to put these techniques into practical use in view of the processing cost and the processing speed.
Japanese Patent Application Laid-open Publication No. 10-320772 discloses a method of manufacturing a magnetic recording medium having isolated magnetic fine particles formed on a substrate by lithography technology using a mask of fine particles having a size of several nanometers to several micrometers, which are two-dimensionally arrayed on a substrate. The method provides a cheap manufacturing method of a patterned media.
A method of ordering fine particles two-dimensionally on a substrate is proposed in, for example, S. Hung et al., Jpn. J. Appl. Phys., 38 (1999) pp. L473-L476. It is proposed that a substrate is coated with fine particles covered with long-chain alkyl groups so as to permit a relatively uniform single particle layer to be formed to cover a large area by utilizing autoagglutination of the fine particles during drying.
Also known is a method of forming a regular array structure on a substrate by utilizing a self-ordering phase separation structure formed by a block copolymer, as reported in, for example, M. Park et al., Science 276 (1997) 1401. It is reported that, in a block copolymer such as polystyrene-block-polybutadiene or polystyrene-block-polyisoprene, it is possible to leave the polystyrene block alone by ozone treatment, and to form a structure of holes or a line-and-space on the substrate by using the left polystyrene block as an etching mask.
In a film-forming method in which self-ordering particles such as fine particles or block copolymer are arrayed two-dimensionally on a substrate, it is possible to obtain a structure in which the self-ordering particles are microscopically arrayed to form a lattice. However, many defects and crystal boundaries are present macroscopically so as to form a lattice directed at random, resulting in failure to achieve practical recording/reproducing.
Also, in the conventional magnetic recording medium having a uniform structure, signals are written at a predetermined interval. Therefore, even if a write error takes place, the recording cells are rendered defective only partly, making it possible to read out the written information at the same time interval in the entire system. On the other hand, when it comes to the patterned media in which the recording cells are formed in advance, it is necessary to perform the processing such that the distances between the adjacent recording cells are rendered constant. Even if it is possible to manufacture a patterned media utilizing the self-ordering particles, it is necessary to form a single regular array free from an internal disturbance or defect in the entire region. However, where ordering processes have taken place from two different sites within the same region, a regular triangular lattice is formed inside each of the self-ordering array. However, the lattice position of one of these two self-ordering arrays does not match with the lattice position of the other self-ordering array. As a result, discontinuity of the lattices is generated in the connecting area of the adjacent self-ordering arrays. Since the read interval of the recording cells differs in the discontinuous portion of the lattices, reproduction of information is rendered difficult. As described above, a region where the array is disturbed is generated as a defect inherent in the recording medium utilizing the self-ordering array, with the result that it is necessary to establish a method of avoiding read errors for using the particular recording medium.
It should also be noted that the track density is increased with increase in the recording density so as to make it very difficult to write servo marks for tracking. A method of achieving a high track density is proposed in, for example, Japanese Patent Application Laid-open Publication No. 6-111502. It is proposed that a servo pattern for tracking is formed in advance in the disk as a physical irregular pattern. In this method, formed is a track close to a true circle, making it possible to increase the track density, compared with the conventional HDD. However, when it comes to a high recording density such as 100-giga(G) bpsi to 1-tera(T) bpsi, it is difficult to form the servo pattern by cheap lithography. Further, in the recording medium utilizing the self-ordering, a regular array structure inherent in the self-ordering particles is formed in the track. It follows that it is impossible for the conventional tracking method to access the recording cells formed of self-ordering particles.
As described above, patterned media are an effective means for realizing a high recording density of the order of Tbpsi. However, a method that permits the manufacture of a pattern with a low cost and with a high throughput has not yet been established. Also, the method using the self-ordering of a material permits the manufacture of a pattern with a low cost and with a high throughput. However, a medium having an entirely arrayed structure to permit access to recorded data has not yet been obtained.